This invention relates to sodium hydrosulfite compositions stabilized to prevent or minimize hydrogen sulfide production when bleaching aqueous wood pulp compositions therewith, and to a process for their use.
Presently, wood pulp is reductively bleached with hydrosulfite solutions, particularly zinc hydrosulfite or sodium hydrosulfite. Zinc hydrosulfite has been the preferred composition for bleaching wood pulp. Coincidentally, little or no hydrogen sulfide was evolved during the bleaching process when zinc hydrosulfite was used. However, the use of zinc hydrosulfite is undesirable from an ecological standpoint since zinc ions are present in the process effluent and are currently disposed of into the surrounding environment. In contrast, sodium hydrosulfite does not present this ecological problem since the sodium ion present in the discarded effluent does not adversely affect the surrounding environment. However, its use has occasionally presented both a nuisance and a health hazard to working personnel due to the hydrogen sulfide which can be formed under some conditions of bleaching and subsequent processing. In addition hydrogen sulfide severely degrades copper-based apparatus employed in downstream paper forming apparatus including copper screening on which the sheet is formed.
It is also known that hydrogen sulfide formation increases in an aqueous system containing wood pulp and sodium hydrosulfite when the pH of the solution is decreased. However, it is not feasible to control hydrogen sulfide production merely by adjusting the solution pH since wood pulp bleaching should be carried out at a pH of about 4.5 and 5.5 in order to optimize bleaching quality and efficiency.
Accordingly, it would be highly desirable to provide a means for preventing hydrogen sulfide formation in sodium hydrosulfite solutions in the presence of wood pulp wherein the quality of bleaching is not adversely affected.